Online Tools

    Radiator Size Calculator

    Estimate room radiator output in BTU/hr and watts, then calculate radiator sections or panel radiators from the rated output.

    Radiator type
    Windows
    This rule-of-thumb estimate starts from BTU/hr per square foot. Radiator output depends on flow and return temperatures, so use the manufacturer's output for your actual system conditions.

    Estimate radiator output for a room

    Radiator sizing SERPs expect the required heat output first, usually in BTU/hr with watts alongside it. This calculator gives a square-foot rule-of-thumb room estimate and then divides by the rated output of a section or panel radiator.

    Qbase — base heat output in BTU/hr, Aft2 — room area in square feet, 35 — rule-of-thumb BTU/hr per square foot.

    BTU/hr — heating output, W — watts, 3.412142 — conversion factor.

    N — required sections, Qreq — adjusted room output, Psection — rated output of one section.

    Where the estimate stops

    • Manufacturer output depends on flow temperature, return temperature and room temperature difference.
    • Exterior walls, glazing, ceiling height and climate can change the true heat loss.
    • A code-compliant HVAC or hydronic design needs a Manual J or equivalent heat-loss calculation.
    • Oversizing can reduce control quality; undersizing can leave the room cold in design weather.
    Not a Manual J calculation
    Use this as a rough radiator-shopping estimate. Building envelope, infiltration, ventilation, design temperature and hydronic system temperature require a qualified load calculation.

    Frequently Asked Questions

    Sources and References

    Calculations are based on the listed reference sources. Links open in a new tab.

    Updated:

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